Trinuclear and tetranuclear cyclopentadienyl vanadium carbonyl clusters: unusual carbonyl groups in Herrmann's (C5H5)4V4(CO)4 exhibiting low CO stretching frequencies

Abstract

The pyrolysis of Cp₂V₂(CO)₅ in tetrahydrofuran solution is reported by Herrmann and co-workers to give a tetranuclear Cp₄V₄(CO)₄ cluster as a major product and the trinuclear Cp₃V₃(CO)₉ as a minor product. Neither of these products has been characterized structurally. A theoretical study of the Cp₄V₄(CO)₄ system reveals a complicated energy surface having 10 structures within 25 kcal mol⁻¹ of the global minimum encompassing singlet, triplet, and quintet spin states. The lowest energy Cp₄V₄(CO)₄ structures are nearly degenerate (within 1 kcal mol⁻¹) triplet and singlet structures with a central V₄ distorted tetrahedron with a μ₃-CO group bridging each face. Higher energy singlet and triplet Cp₄V₄(CO)₄ structures are also found with a central V₄ butterfly and an unusual η²-μ₄-CO group bridging all four vanadium atoms through four V–C bonds and one V–O bond. None of the ten Cp₄V₄(CO)₄ structures has any terminal CO groups, which is totally different from the Cp₄V₄(CO)₄ structure with exclusively terminal CO groups suggested from the experimental work. The lowest energy Cp₃V₃(CO)₉ structure has a central equilateral V₃ triangle and all terminal CO groups. However, this Cp₃V₃(CO)₉ structure is disfavored by ∼34 kcal mol⁻¹ relative to fragmentation into the stable compounds CpV(CO)₄ + Cp₂V₂(CO)₅ consistent with its low yield in the Cp₂V₂(CO)₅ pyrolysis.